It's an old question but I guess I found an easy answer.
Its from everyone's favourite media player VLC. The best part is that you can change only what you want (audio or video separately) it doesn't re-encode everything and saves time if you only want to re-encode only one of them.
Press ctrl+R OR click on top left media--->convert/save
Then click on Convert/Save
Choose a video format. Or if you don't want to change that then tick keep original video track
Choose a audio format. Or if you don't want to change that then tick keep original audio track
Select save.
Select convert/save.
Wait!!! and You are done. enjoy!!!